By default, when you print opaque, overlapping colors, the top color knocks out the area underneath. You can use overprinting to prevent knockout and make the topmost overlapping printing ink appear transparent in relation to the underlying ink. The degree of transparency in printing depends on the ink, paper, and printing method used. Consult your print shop to determine how these variables will affect your final artwork.
Where colors printed from separate plates overlap or adjoin one another, press misregistration can cause gaps between colors on the final output. To compensate for potential gaps between colors in
artwork, print shops use a technique called trapping to create a small area of overlap (called a trap) between two adjoining colors. You can use a separate, dedicated trapping program to create traps automatically, or you can use Illustrator to create traps manually.
You may want to overprint in the following situations:
Overprint black ink to aid in registration. Because black ink is opaque (and usually the last to be printed), it doesn’t look much different when printed over a color as opposed to a white background. Overprinting black can prevent gaps from appearing between black and colored areas of your artwork.
Overprint when the artwork does not share common ink colors and you want to create a trap or overlaid ink effects. When overprinting process color mixes or custom colors that do not share common ink colors, the overprint color is added to the background color. For example, if you print a fill of 100% magenta over a fill of 100% cyan, the overlapping fills appear violet, not magenta.
After you set overprinting options, you should use the Overprint Preview mode (View > Overprint Preview) to see an approximation of how the overprinting colors will print. You should also carefully check overprinted colors on separated artwork using integral proofs (where each separation is shown in register on a single piece of paper) or overlay proofs (where the separations are shown in register on separate plastic sheets stacked on top of each other).
If you use the Overprint option on a 100% black stroke or fill, the black ink may not be opaque enough to prevent the underlying ink colors from showing through. To eliminate the show-through problem, use a four-color (rich) black instead of a 100% black. Consult with your print shop about the exact percentages of color to add to the black.
To overprint all black in your artwork, select the Overprint Black option in the Print dialog box when you create color separations. This option works on all objects that have black applied through the K color channel. However, it does not work for objects that appear black because of their transparency settings or graphic styles.
You can also use the Overprint Black command to set up overprinting for objects that contain a specific percentage of black. To use the Overprint Black command:
To remove overprinting from objects that contain a specific percentage of black, select Remove Black instead of Add Black in the Overprint Black dialog box.
In most cases, only separation devices support overprinting. When you print to a composite, or when your artwork contains overprinted objects that interact with transparency objects, you choose to simulate or discard overprinting.